In one regard, those numbers signal an improvement for Donnelly, who had failed to crack $400,000 in each of the three previous quarters.

Democrats have long believed he stands a better chance against Mourdock than he would have against Lugar, and his fundraising reflects an uptick in donations now that the stage is set for what they see as a showdown between a moderate Democrat and a tea party Republican.

Mourdock's camp, meanwhile, has refuted the notion that Donnelly fits into Indiana's political mainstream. They cite his vote for President Barack Obama's health care reform law, saying he wants to "cherry-pick" the portions of the law he publicly addresses.

And with their nearly two-to-one fundraising advantage added to outside spending from groups such as the Club for Growth and FreedomWorks, which supported Mourdock in the primary, he maintains the financial advantage.

The candidates haven't filed their second-quarter campaign finance reports with the Federal Election Commission, so the amount of money they have on hand is not yet available.